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Similar a Exafs analysis of PbZrO3 amorphous powders prepared by an alkoxide-based sol-gel route under different hydrolysis conditions (20)
Exafs analysis of PbZrO3 amorphous powders prepared by an alkoxide-based sol-gel route under different hydrolysis conditions
![PbZrO 3 (PZ) powders have been prepared by an alkoxide-based sol-gel route, starting from lead acetate, zirconium n-butoxide, and n-butanol as a solvent, and hydrolysed with different molar ratios of water in neutral and alkaline medium. After drying (150 o C/12h) the hydrolysis product is amorphous as determined by XRD and only after heating (700 o C/1h) it crystallizes as perovskite phase * . In the present work we used XAFS for studying the local environment of metallic atoms in PZ dried powders as function of hydrolysis conditions in order to determine the short range order and the degree of homogeneity on the atomic level. Introduction Elena-Daniela Ion 1, 2 , Barbara Malič 1 , Iztok Arčon 3,1 , Alojz Kodre 4,1 , Marija Kosec 1 1 Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia, 2 National Institute of Materials Physics, P.O. Box MG-7, Bucharest-Magurele, Romania 3 University of Nova Gorica , Vipavska 13, 5000 Nova Gorica, Slovenia, 4 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia EXAFS analysis of PbZrO amorphous powders prepared by an alkoxide-based sol-gel route under different hydrolysis conditions 3 XANES: formal oxidation state coordination chemistry EXAFS: distances, coordination number , type of the neighbours XANES EXAFS k, k 2 or k 3 - space FT R - space Pb ( OAc ) 2 Zr ( OR ) 4 solvent ( R OH) - ( Pb -OAc) + ( RO -Zr) - - ( Pb -O- Zr ) - + R OAc - ( Pb -OAc) + ( RO -Zr) - -( Pb - OAc Zr - OR )- Precursor solution Hydrolysis Drying 150 o C/12h Heatin g 700 o C/1h Ceramic powder Pb ( Zr ) ( O ) x ( OAc ) 2-y ( OR ) 4-y Reflux and Distillation By-product: ester ( R OAc ) __ Hydrolysis conditions __ R w = 2, 5 or 15 (Neutral medium pH=6) R w =15 (Alkaline medium pH=11 ) R w – molar ratio H 2 O/Zr(OR) 4 pH - PZ – R w Z – R w e.g. PZ 2 , 11 PZ 15 Z 5 Experimental __ Alkoxide-based sol-gel synthesis __ __ X-ray Absorption Fine Structure __ The XAFS measurements were performed on pellets prepared from the dried powders and a dilutant, b oron n itride (BN). The amount of each sample was calculated for a total absorption d ~ 2. Zr K-edge and Pb L3-edge were measured at ROEMO II (X1) station , HASYLAB, Hamburg. All EXAFS spectra were analysed by IFEFFIT program packages using FEFF6 for ab i nitio calcu l ation of scatt e ring paths. Ac = CH 3 CO- R = CH 3 (CH 2 ) 3 - M = Zr Zr ( OR ) 4 solvent ( R OH) Hydrolysis ,[object Object],[object Object],[object Object],[object Object],Drying 150 o C/12h Heatin g 500 o C/1h PZ Z Zr - XANES Pb - XANES The Zr XANES spectra of amorphous powders are markedly different in comparison to the spectrum of the PbZrO 3 , indicating different local simmetry of Zr atom s . Pb XANES shows that t he coordination of Pb atom in amorphous powders is different from th at in the perovskite phase . The k 3 weighted FT of Zr K-edge EXAFS spectra , _ exp., . . . . fit Zr - EXAFS Pb - EXAFS The k 3 weighted FT of Pb L 3 -edge EXAFS spectra , _ exp., . . . . fit - Zr neighbourhood - modelled by two separate shells. - 1 st shell - described by a bout seven O atoms at two slightly different distances: 2.12 Å and 2.25 Å in both PZ and Z 5 powders - 2 nd shell in PZ - five Zr atoms at 3.46 Å and one Pb atom at 3.59 Å. - 2 nd shell in Z5 - ~ six Zr atoms at 3.44 Å -FT of Pb L3-edge spectra of dried PZ powder prepared with different hydrolysis conditions are similar. - The 1 st shell is composed of two O atoms at two slightly different distances. - In the 2 nd shell one zirconium atoms is present at around 3.59 Å. Zr 4+ (CN=6) Amorphous powders, dried at 150 o C Crystalline powder, heat treated at 700 o C/1h Crystalline powder, heat treated at 700 o C/1h Amorphous powders, dried at 150 o C 1:1 Zr – O – Pb 1 st shell ~5 O ~2 O 2 nd shell ~5 Zr ~1 Pb Central atom Zr 2.12 2.25 3.46 3.59 1 2 3 4 R ( Å ) Central atom Pb 1 st shell ~1 O ~1 O 2 nd shell ~1 Zr 2.19 2.32 3.59 1 2 3 4 R ( Å ) Zr EXAFS: Pb :EXAFS From both sides analysis we found that Zr is linked with one Pb atom and Pb is linked with one Zr atom. It is very probable that an O bridge connects these two atoms in the stoichiometry 1:1 and that all prepared samples are stoichiometric at atomic level. * Ion E.D., et. all., Journal of European Ceramic Society 27, 13-15, (2007) 4349 – 4352. . An alkoxide-based sol-gel route has been used to prepare PbZrO 3 ( PZ ) . The precursor solution is hydrolysed in neutral and alkaline medium with different R w . The XAFS investigations show that local environment of Zr and Pb atoms is not affected by the hydrolysis condition and is totally different from the environment in the perovskite phase. The Zr neighbourhood in PZ resembles that of amorphous precursour for ZrO 2 (Z5). The large number of Zr atoms which are found in the 2 nd coordination shell in Zr – EXAFS spectra is an evidence of the preferred homocondensation of Zr species. However a link between Zr and Pb is established in the stoichiometry 1:1 which is not destroyed by further condensation of Zr. This work was supported by the Slovenian Research Agency research programmes P1-0112 and P2-105 , and by DESY and the European Community under the FP6 Programme "Structuring the European Research Area" contract RII3-CT-2004-506008 (IA-SFS). Access to synchrotron radiation facilities of HASYLAB (project II-04-065) and expert advice on beamline operation by Edmund Welter of HASYLAB is acknowledged. SICER (G1MA – CT – 2002 - 04029) is also aknowledged for the financial support. The authors wish to thank Jana Padeznik Gomilsek for help in XAFS measurements and Laura Lazarini for help in the experimental work. Conclusions Acknowledgments Results & Discussions](https://image.slidesharecdn.com/exafsanalysisofpbzroamorphouspowderspreparedby-090626202204-phpapp02/85/Exafs-analysis-of-PbZrO3-amorphous-powders-prepared-by-an-alkoxide-based-sol-gel-route-under-different-hydrolysis-conditions-1-320.jpg)
